How to Read a Peptide Certificate of Analysis

A Certificate of Analysis, commonly called a COA, is one of the most useful tools for evaluating a research peptide. But simply seeing the words “third-party tested” or “99% pure” does not tell the whole story.

A meaningful peptide COA should help answer several different questions about the material being tested. At USA Peptide Labs, we believe three areas deserve particular attention:

Identity. Purity. Endotoxins.

Each measures something different, and understanding the difference makes a COA much easier to interpret.

1. Identity: Is It Actually the Correct Peptide?

This is the first question any analytical report should answer.

A sample can appear extremely pure on a chromatogram and still not necessarily be the compound stated on the label. Scientific literature on synthetic-peptide characterization specifically notes that a single dominant HPLC peak does not by itself establish that the peak represents the intended peptide.

That is why identity testing matters.

For peptides, identity is commonly evaluated using mass spectrometry, frequently LC-MS. Mass spectrometry measures the molecular mass of the compound and compares the observed result with the expected mass of the peptide. More detailed MS/MS analysis can provide additional information about peptide sequence and structure.

What to look for on the COA

Look for terms such as:

• LC-MS
• Mass Spectrometry
• Molecular Mass
• Expected Mass
• Observed Mass
• Identity Confirmed

The important question is simple:

Does the analytical result support the identity of the peptide listed on the vial?

Think of identity testing as checking the name on the driver’s license before worrying about everything else.

2. Purity: How Clean Is the Peptide?

Once identity has been established, the next question is purity.

Peptide purity is commonly evaluated using High-Performance Liquid Chromatography (HPLC) or a related chromatographic method.

HPLC separates the components within a sample. The resulting chromatogram normally displays peaks representing compounds detected during the analysis. The desired peptide should account for the overwhelming majority of the relevant chromatographic signal when a highly purified preparation is being evaluated. Analytical HPLC is widely used for assessing relative peptide purity.

For example, a report might state:

Purity: 99.3%

That generally means approximately 99.3% of the integrated chromatographic signal under that analytical method was attributed to the principal peptide peak.

What to look for

A useful purity report should ideally include:

• The reported purity percentage
• The analytical method, such as HPLC
• A chromatogram
• Identification of the primary peak
• Laboratory and sample information
• Lot or batch identification

At USA Peptide Labs, 99%+ verified purity is the standard we want customers to be able to see documented rather than simply take on faith.

One important distinction

99% purity does not automatically mean that 99% of everything inside the vial by weight is peptide.

Lyophilized peptide preparations can also contain water, counterions, salts or other components. Analytical purity and absolute peptide content are related but technically different measurements. Peptide reference-standard research therefore distinguishes chromatographic purity from measurements of actual peptide content.

For the average person reading a peptide COA, however, the important takeaway is straightforward:

Purity tells you how clean the peptide preparation appears analytically. Identity tells you whether the primary compound is actually the peptide you expected.

You want both.

3. Endotoxins: The Test People Often Overlook

The third area is bacterial endotoxin testing.

Endotoxins are associated with Gram-negative bacteria and can remain present even when viable bacteria themselves are no longer present. For that reason, endotoxin testing is its own analytical category and is not replaced by HPLC purity or mass-spectrometry identity testing. FDA guidance specifically addresses bacterial endotoxin testing as a distinct quality-control issue for applicable products.

Historically, the best-known bacterial endotoxin test has been the LAL assay — Limulus Amebocyte Lysate. Newer pharmacopeial approaches also allow appropriately validated recombinant-reagent methods. USP added Chapter 86 addressing bacterial endotoxin testing using recombinant reagents, and FDA updated its guidance in 2026 to accommodate these newer methods.

What should you look for?

Endotoxin results are commonly reported in Endotoxin Units, usually abbreviated EU, with results expressed relative to a volume or amount of material, depending on the test.

Instead of simply seeing:

Endotoxin: PASS

a more informative report provides an actual result or limit, such as:

< X EU/mg

or another clearly identified measurement appropriate to the assay.

The laboratory should also identify the test method and sample being evaluated.

“Endotoxin-Free” vs. Endotoxin Tested

This distinction is worth understanding.

Scientifically, it is generally more precise to say that a sample was tested for bacterial endotoxins and found below a specified analytical or acceptance limit than to imply that an assay has demonstrated the absolute absence of every endotoxin molecule.

That’s why the actual laboratory result matters.

Why All Three Matter Together

Imagine receiving a COA that says:

Purity: 99.6%

That sounds excellent.

But what if there is no identity test?

You know the dominant compound appears highly pure, but you haven’t independently confirmed what that compound actually is.

Now imagine the identity and purity both check out—but endotoxin testing was never performed.

That is a completely different unanswered question.

This is why we view the three tests as complementary:

IDENTITY
Is it the correct peptide?

PURITY
How clean is the peptide preparation?

ENDOTOXINS
What level of bacterial endotoxin was detected?

A strong analytical program does not substitute one for another.

Five Things to Check Before Trusting a COA

When reviewing a peptide COA, take a few seconds to verify:

• Does the product and batch number match the vial you’re evaluating?
• Was identity independently tested?
• Was purity independently measured?
• Was bacterial endotoxin testing performed?
• Can you see the actual results rather than simply a generic “tested” badge?

The laboratory itself also matters. Look for identifiable laboratory information, testing dates, analytical methods and batch-specific results.

A COA should provide information you can verify, not merely another marketing claim.

The Bottom Line

You do not need to be an analytical chemist to understand the most important parts of a peptide COA.

Remember three questions:

Is it really what the label says it is?
That is identity.

How clean is it?
That is purity.

Was it tested for bacterial endotoxin contamination?
That is endotoxin testing.

Those three measurements provide a much more complete picture than a purity percentage alone.

At USA Peptide Labs, our goal is to make analytical testing understandable and accessible rather than expecting customers to blindly trust a number printed on a website.

Every Body Is A Lab.

USA Peptide Labs products are intended strictly for laboratory research use and are not intended for human or veterinary use.